In a typical optical link that is transporting many radio frequency (RF) carrier channels, there exists composite second order distortion (CSO) components. The CSO components result from inherent nonlinearity of the link components, laser chirp interactions, and other effects. In one application, for example, CSO distortions primarily appear at +/−1.25 Mhz offsets from either side of the carrier center frequencies, and in another application the CSO distortions appear +/−750 kHz offsets from the carrier when the frequencies are spaced differently than 6 Mhz. These components follow the (fx−fy) and (fx+fy) rule for every carrier frequency, whereby fx and fy are combinations of any two different frequencies in the total band transmitted.
Another way in which even-order CSO distortion manifests itself is by the (fn+1−fn) rule. This rule states that a distortion product is generated for every pair of RF frequencies that are adjacent to each other. A typical optical system may operate with cable television (CATV) National Television System Committee (NTSC) RF carrier frequencies. The majority of these carrier frequencies are nominally equally spaced at 6.00 Mhz intervals, within a tolerance of less than +/−30 Khz. The actual spacing between carriers depends on the absolute frequency accuracy of each individual carrier. This results in many distortion products that fall within the 6.00 Mhz+/−30 Khz frequency range (e.g. when detected using a photodiode). For a system transmitting N carriers, there will be (N−1) of these CSO 6.00 Mhz difference signals, or ‘beats’. For other channel plans, such as European PAL format, the channel spacing will be different at 7.00 Mhz or 8.00 Mhz, with corresponding CSO beat products.
A measure of the energy in all these 6.00 Mhz beat frequencies is indicative of the even-order distortion present on the optical link. If these 6.00 Mhz components are monitored and a mechanism exists that reduces their magnitude, then the other CSO components falling at +/−1.25 Mhz on either side of the carriers may also be reduced.
U.S. Pat. No. 6,687,466 describes automatic distortion cancellation in an optical transmitter.